1. Field of the Invention
The present invention relates to stable inkjet inks comprising a yellow pigment in a non-aqueous medium.
2. Description of the Related Art
Pigment dispersions are made using a dispersant. A dispersant is a substance for promoting the formation and stabilization of a dispersion of pigment particles in a dispersion medium. Dispersants are generally surface-active materials having an anionic, cationic or non-ionic structure. The presence of a dispersant substantially reduces the dispersing energy required. Dispersed pigment particles may have a tendency to re-agglomerate after the dispersing operation, due to mutual attraction forces. The use of dispersants also counteracts this re-agglomeration tendency of the pigment particles.
The dispersant has to meet particularly high requirements when used for inkjet inks. Inadequate dispersing manifests itself as increased viscosity in liquid systems, loss of brilliance and/or hue shifts. Moreover, particularly good dispersion of the pigment particles is required to ensure unimpeded passage of the pigment particles through the nozzles of the print head in an inkjet printer, which are usually only a few micrometers in diameter. In addition, pigment particle agglomeration and the associated blockage of the printer nozzles has to be avoided in the standby periods of the printer.
Many polymeric dispersants contain in one part of the molecule so-called anchor groups, which adsorb onto the pigments to be dispersed. In a spatially separate part of the molecule, polymeric dispersants have polymer chains sticking out whereby pigment particles are made compatible with the dispersion medium, i.e. stabilized.
The properties of polymeric dispersants depend on both the nature of the monomers and their distribution in the polymer. Polymeric dispersants obtained by statistically polymerizing monomers (e.g. monomers A and B polymerized into ABBAABAB) or by polymerizing alternating monomers (e.g. monomers A and B polymerized into ABABABAB) generally result in poor dispersion stability. Improvements in dispersion stability were obtained using graft copolymer and block copolymer dispersants.
In the design of polymeric dispersants for pigment dispersions, dispersants which contain a polyester chain moiety derived from one or more hydroxycarboxylic acids or lactones thereof have been known for a long time. Early examples of such dispersants are disclosed in EP 0158406 A (ICI) for dispersing finely divided particles of a magnetic material in an organic liquid.
These are generally of two distinct chemical types. In the first type, the hydroxycarboxylic acid or lactone is polymerized in the presence of an alkylcarboxylic acid as polymerization terminating group to give a polyester chain having a free carboxylic acid which is then reacted with an amine such as polyethyleneimine.
In the second type of dispersant, the hydroxycarboxylic acid or lactone is polymerized in the presence of an aliphatic alcohol as polymerization terminating group to give a polyester chain having a free hydroxyl group which is subsequently converted to a phosphate ester. Early examples of such dispersants are disclosed in EP 0164817 A (ICI).
More recently, the properties of such dispersants have been improved by branching the alkylene group of the polyester chain as disclosed in U.S. Pat. No. 6,197,877 (ZENECA) or by using polymerization terminating groups containing a branched aliphatic chain as disclosed in US 2003181544 (AVECIA).
Generally pigments have a non-polar surface. In aqueous inkjet inks, the polymeric dispersants generally contain hydrophobic anchor groups exhibiting a high affinity for the pigment surface and hydrophilic polymer chains for stabilizing the pigments in the aqueous dispersion medium. The preparation of thermally stable dispersions with submicron particles is often more difficult in the case of non-aqueous inkjet inks where both the pigment surface and the dispersion medium are non-polar. Especially quinacridone pigments and yellow pigments, such as C.I. Pigment Yellow 120, C.I. Pigment Yellow 213 and C.I. Pigment Yellow 150, pose difficulties.
For consistent image quality, the inkjet ink should be capable of dealing with elevated temperatures during transport of the ink to a customer and also during inkjet printing where the inkjet ink is generally heated to a temperature above 40° C.
The consistency of image quality also depends on the consistent quality of the raw materials used to prepare the pigment dispersions. For a number of pigments, the quality changes so drastically from batch to batch in production that some batches become unsuitable for producing inkjet inks. One method used by ink manufacturers to handle this raw material inconsistency is to order samples of different new production batches and to evaluate if they are suitable for preparing inkjet inks. This evaluation is not only a time consuming and costly activity but also restricts the flexibility in ink manufacturing requiring the availability of larger stocks to guarantee deliverability of ink.
Due to their insoluble character, the quality of colour pigments tends to change from batch to batch in production. Differences in production batch quality are particularly often observed in producing non-aqueous yellow pigmented inkjet inks. This can lead to inconsistent image quality and even to malfunction of inkjet printers.